Polylactide (PLA) is a polymeric material having optical characteristics, which has been used as a biodegradable or bioabsorbable medical material for surgical suture thread, microcapsules for injection or the like, and recently, is employed as an environmental-friendly biodegradable material which can be used in the production of various high-molecular products such as packaging materials, home appliances, office supplies, vehicle interior materials, or the like.
In order for polylactide to be used for the above usage, it is required to have a high optical purity (D-type or L-type optical isomer) and have high molecular weight. For this, lactide, which is a monomer used for producing polylactide, should have high optical purity and chemical purity.
The following Reaction Chart 1 shows routes for producing lactide from lactic acid or lactic acid ester. In case of using lactic acid as a starting material, lactide can be produced through a course of Route-1/Depolymerization or a course of Route-4. In case of using lactic acid ester as a starting material, lactic can be produced through a course of Route-2 or a course of Route-3/Depolymerization.

The course of Route-1/Depolymerization is a conventional method for producing lactide by using lactic acid as a starting material, wherein lactide is obtained via a two-step process consisting of firstly polymerizing lactic acid under reduced pressure to produce an oligomer or prepolymer in a molecular weight of about 500˜5,000, and then depolymerizing the produced prepolymer with the flow of inert gas under a reduced pressure (U.S. Pat. No. 5,274,073; U.S. Pat. No. 5,247,059; U.S. Pat. No. 5,274,127; U.S. Pat. No. 6,277,951). NatureWorks (a company in USA) commercially produces L-type PLA bioplastic in a scale of about 140 thousand tons a year by a manner consisting of producing lactide from L-type lactic acid via said two-step process, and then subjecting the resulting lactide to a condensation polymerization reaction in a polymerization reactor. Recently, Purac (a company in Netherlands) has also built a plant in Thailand and started a commercial production of lactide via the above two-step reactions.
However, in case of going through the two-step process, the products such as prepolymer and lactide are deteriorated when they reside for a long period of time within the reactor, and thus there is a problem that the amount of byproducts produced such as meso-lactide increases during the production of L-type or D-type lactide. The two-step process of Prepolymerization-Depolymerization of lactic acid has problems that part of the prepolymer or oligomer which is the reactant in the Depolymerization step is not decomposed to lactide and further polymerizes or forms a waste material mixed with the catalyst component included in the prepolymer. Also, the two-step process involves problems that a vacuum pump for making a high vacuum is required and the reaction device is complex to make the device cost high.
The course of Route-2 or Route-3/Depolymerization is a method using lactic acid ester as a starting material. There are several references disclosing a process via Route-2 for the production of lactide directly or by one-step from lactic acid ester, or a process via Route-3 and Depolymerization for the production of lactide through prepolymer from lactic acid (e.g. Korean Patent Application No. 10-2009-0043985, Japanese Patent Laid-Open No. 1999-036366, Japanese Patent Laid-Open No. 1993-286966, and Japanese Patent Laid-Open No. 1994-031175).
Finally, the process via Route-4 relates to a method for producing lactide directly or by one step from lactic acid as a starting material. A method for the production of lactide by this process or a catalyst for the production thereof has ever been suggested, but the suggested method and catalyst have failed to provide satisfactory results in conversion ratio and selectivity when considering the process efficiency.
U.S. Pat. No. 5,332,839 suggested a direct production of lactide by a process via Route-4. For example, it reports that lactide can be produced directly from lactic acid without going through a prepolymer by gasifying lactic acid at a high temperature of at least 200° C. and then reacting it in a fixed layer charged with a solid acid such as Al2O3. In case of using a solid acid catalyst such as Al2O3, however, there are limitations such that the productivity of lactic acid oligomer is high, the yield of lactide is low, carbon monoxide is generated by the degradation of lactic acid during the reaction, and the life of the catalyst is short.
U.S. Pat. No. 5,138,074 also suggested a direct production of lactide by a process via Route-4. For example, it discloses an example for producing lactide directly from lactic acid in the presence of SnO. However, it does not disclose results such as conversion ratio of lactic acid, selectivity of lactide, production ratio of oligomer byproduct, and thus it is difficult to refer to the catalyst and reaction in detail.
Many methods for producing lactide by using lactic acid ester as a starting material (Route-2 and Route-3) and methods for producing lactide by a two-step process by using lactic acid as a starting material (Route-1 and Depolymerization) have been developed. The method for producing lactide directly or by one step from lactic acid as a starting material (Route-4) has not been employed in a commercial process due to the absence of a suitable catalyst, although it has an advantage that the process is simple when compared with the two-step process by the Prepolymerization/Depolymerization of lactic acid (Route-1 and Depolymerization).